The present invention relates generally to a taking optical system adapted to be connected to an eyepiece of a viewing optical system for taking images, and more particularly to a taking optical system adapted to be connected to an eyepiece of an endoscope to take images on TV cameras.
With an endoscope having an eyepiece lens such as a hard endoscope, and a fiber scope, a detachable endoscope TV camera is positioned in the rear of the eyepiece lens to guide images in the body cavity, obtained through the endoscope, to an imaging portion for viewing on TV.
For instance, referring to a TV camera for the urinary organs, a camera head 5 having a taking lens 7 is mounted in the rear of an eyepiece lens 4 of a hard endoscope 1, as shown typically in FIG. 1, so that images in the body cavity can be viewed on a TV monitor 9 via that camera head 5. Note here that the hard endoscope 1 has an objective lens 2, a relay lens 3, etc. inside, and the camera head 5 has a field direction turning prism 6, a taking lens 7, a CCD or other imaging device 8, etc. inside. More specific construction of the camera head is shown typically in Patent Publication 1.
So far, a typical imaging device used with a TV camera has been designed such that the best shading performance is obtainable when off-axis chief rays are vertically incident on an imaging plane. An endoscope in general, and a hard endoscope used for surgical operations or the like in particular, has often an exit pupil position of the eyepiece lens 4 jutting about 10 mm from an eyepiece in consideration of convenience of visual observation. In a conventional endoscope TV camera, therefore, the exit pupil of an endoscope eyepiece optical system is in alignment with the front focus position of a taking lens to form a taking optical system telecentric on the image side, whereby off-axis chief rays are vertically incident on the imaging plane.
In recent years, the environment surrounding imaging devices has been as mentioned below.
First, they grow smaller and smaller.
Second, there are specifications with which the shading performance for the angle of incidence of rays is optimized.
Referring here to the first consideration, a typical prior art is the following (A), and a more recently developed imaging device is the following (B):
(A) an imaging device designed such that when off-axis chief rays are vertically incident on an imaging plane (when the exit pupil position of a taking lens is at infinity with respect to the imaging plane), the best shading performance is achievable: this will hereinafter be called the imaging device optimized for vertical incidence (FIG. 11(a)), and
(B) an imaging device designed such that when off-axis chief rays are incident on the imaging plane at an angle of divergence (when the exit pupil position of a taking lens is on the object side with respect to the imaging plane), the best shading performance is achievable: this will hereinafter be called the imaging device optimized for oblique incidence (FIG. 11(b)).
TV camera heads are preferably small in size and light in weight so as to be not a burden on the operation of endoscopes. To this end it is preferable to use a small-format imaging device in view of mechanical design. However, there are the following problems 1 and 2 involved in the mechanical design:
(1) For a camera head using a small-format imaging device, it is required to make the focal length of a taking optical system short depending on the size of the imaging device. However, because the exit pupil position of an eyepiece lens and the spacing between the eyepiece lens and the taking optical system remain invariable, the exit pupil position of the taking lens is going to lie on the image side with respect to an image side (to have a plus value). In this case, off-axis chief rays are at an angle of convergence with respect to the imaging plane (FIG. 12). The matching of the exit pupil of the taking lens and the shading performance of the imaging device goes worse with the result that there is shading. Shading is a problem with both (A) and (B), and that problem gets graver with the imaging device (B) optimized for oblique incidence in particular.
It is here understood that while reference has been made to the TV camera head using a field direction turning member, similar problems also arise when the exit pupil position of the eyepiece lens cannot be proximate to the taking lens of the camera head, as
(a) in the case where the exit pupil position juts less (less than 10 mm) from the eyepiece, and
(b) in the case where members such as a filter and a stop are interposed between the eyepiece lens and the camera head.
(2) As the focal length of the lens becomes short, the back focus of the lens becomes short. As a result, when focus adjustment is implemented by changing the spacing between the taking lens and the imaging device, interference is likely, or it is difficult to provide a sufficient focus range.
Apart from the aforesaid prior art TV camera head, there are such arts as set forth typically in the following patent publications.
[Patent Publication 1]
Utility Model 6-18328
[Patent Publication 2]
JP(A) 2005-99080
[Patent Publication 3]
JP(A) 2006-53218
[Patent Publication 4]
U.S. Pat. No. 4,898,457
The camera head of Patent Publication 2 is to make correction of various aberrations and make sure a back focus in the case of using a small-format imaging device. However, this camera head is of the type wherein the optical axis of an eyepiece lens is in line with that of a taking optical system, and so any prism for turning field directions cannot be located. This publication says nothing specific about the exit pupil of the taking lens and the shading performance of the imaging device.
The camera head of Patent Publication 3 is to improve on the shading performance in view of the exit pupil of a taking lens. However, the camera head is optimized for the aforesaid prior art type (A); it is of the type wherein the optical axis of an eyepiece lens is in line with that of a taking optical system, and so any prism for turning field directions cannot be located.
Patent Publication 4 describes a TV camera lens for a television tube, wherein a field lens fit for each construction of taking lens is located just in front of the television tube. However, this is directed to a TV camera for general shooting, not a camera head connected to an eyepiece lens for use.